If you want to show a trend, as a prediction of future values from present and past known values, use the trend() function.
If you want a best-fit line ala least-squares, use the linest() function.
You can get more info from the OpenOffice.org Calc Help manual.
I found this in re linest: http://justlinux.com/forum/archive/i.../t-145367.html
And this site has OpenOffice.org macros, two of which refer to least-squares fitting: http://sourceforge.net/project/showf...group_id=87718